The invention is based on a priority application EP 02 360 159.4 which is hereby incorporated by reference.
The invention relates to an optical time multiplexer for generating an N Gbit/s output signal from n data modulated input pulse streams with a pulse frequency of N/n GHz, where nεIN (IN=natural series) and n≧2, with a combiner device for passively interleaving the n input pulse streams.
An optical time multiplexer of that type and the related method of signal transmission was described by M. Nakazawa, T. Yamamoto and K. R. Tamura in ECOC 2000, presentation 2.6; see also Electron. Lett. 36, p. 2027–2028 (2000).
The optical time multiplex technique is a method for transmitting a plurality of channels as a single optical signal. For coding, the signals of the channels are interleaved into the single optical signal. The optical demultiplexing by gating of the single optical signal according to a fixed time schedule allows the recovery of the original channel information.
In order to generate e.g. a 160 Gbit/s RZ (return to zero) signal out from four 40 GHz pulse streams, the four 40 Gbit/s RZ pulse streams are passively interleaved in a combiner (coupler) device after introducing proper mutual delay times of 0, ¼, ½ and ¾ bit periods (one bit period=25 ps) to the four signals.
Production tolerances of the equipment and temperature drift usually leads to non-constant optical phases between the four signals which leads to non-constant interference between different bits and thus to drifting and worst-case signal distortion. This must be taken into account by using very short pulses with a 30 dB extinction at the position of an adjacent bit, so the interference can be neglected. However, such short pulses are highly susceptible to dispersion,. Moreover, the non-constant optical phases between the four signals lead to a strong penalty at the receiver after pulse broadening due to dispersion.
Nakazawa et al. used very short pulse widths on the order of 200 fs and a complex pre-chirping technique applying a grating pair, a spatial light modulator and a reverse dispersion fiber to compensate for dispersion during multiplexing.
I. Moita and N. Edagawa, OFC 2002, Presentation TuA4, p. 5–6, showed that good long distance transmission (2000 km) properties of optical time division multiplex (OTDM) signals can be achieved with carrier suppressed RZ signals having a phase shift of 180° between two interleaved channels. However, the generation of carrier suppressed RZ signals suffers from the same difficulties as the generation of time multiplexed signals in general.
It is the object of the invention to present an optical time multiplexer which reduces the drifting penalty of ODTM signals due to multiplexer instability, and which avoids worst-case distortions which are likely in the case of broad RZ pulses with non-constant mutual phase differences, and which grants the possibility to easily generate carrier-suppressed RZ signals or similar modulation formats.